A scintillator single crystal is used for a radiation detector that detects a γ-ray, X-ray, α-ray, β-ray, neutron ray, and the like. Such a radiation detector is being widely applied to medical imaging devices such as a Positron Emission Tomography (PET) device and an X-ray Computerized Tomography (CT) device, various radiation measurement devices in the field of high energy physics, resource exploration devices, and the like. The radiation detector is generally constituted with a scintillator that absorbs the γ-ray, X-ray, α-ray, β-ray, neutron ray, and the like and converts these into plural low-energy photons (scintillation light) and a light-receiving element that receives the luminescence from the scintillator and converts the light into electric signals. In the diagnosis of cancer that uses the Positron Emission Tomography (PET) device, glucose having a property of gathering around cancer cells is mixed with a trace of radioisotope and administered to a patient in advance, the γ-ray emitted from the substance is converted into plural low-energy photons by a scintillator, the photons are converted into electric signals by using a Photodiode (PD), a Silicon Photomultiplier (Si-PM), a Photomultiplier Tube (PMT), or other photodetectors, and the electric signals undergo data processing by using a PC or the like to obtain information such as images, whereby the site of cancer is found. Each of a pair of γ-rays is emitted in a diametrically opposite direction. In the PET device, radiation detectors (constituted with a scintillator and a photodetector) are arranged in a cylindrical shape, the scintillators at two locations that the γ-rays hit emit light, and the photodetectors convert the light into electric signals. All of the electric signals are collected by a circuit in the rear of the device and reconstructed into an image by using software. Even in the radiation detector in high energy physics, the process, in which the scintillator converts radiation into plural low-energy photons, the photons are converted into electric signals by using a Photodiode (PD), a Silicon Photomultiplier (Si-PM), a Photomultiplier Tube (PMT), or other light-receiving elements, and the electric signals undergo data processing by using a PC or the like, is applied in the same manner.
A PD or Si-PM is used for extensive purposes particularly in radiation detectors or imaging instruments. Various PDs are known, and the PD or Si-PM constituted with a silicon semiconductor exhibits high sensitivity to a wavelength of 450 nm to 700 nm, and the sensitivity thereof becomes the highest at around 600 nm. Accordingly, they are used in combination with a scintillator having a peak emission wavelength around 600 nm. For radiation imaging, a combination of a scintillator array and a photodetector array is used. Examples of the photodetector include a position-sensitive PMT and an array of semiconductor photodetectors, that is, a PD array, an Avalanche Photodiode array (APD array), a Geiger-mode APD array, and the like. The photodetector identifies which pixels luminesce in the scintillator array, thereby making it possible to ascertain at which position the radiation enters in the scintillator array.
Therefore, the scintillator appropriate for these radiation detectors are required to have high density and a high atomic number (have a high photoelectric absorption ratio) in view of detection efficiency, and required to emit a large amount of light and have a short fluorescence life time (fluorescence decay time) in view of the need for high-speed response and high-energy resolution. It is also important for the emission wavelength of the scintillator to match up with the wavelength band where the detection sensitivity of the photodetector becomes high.
Currently, as a preferable scintillator applied to various radiation detectors, there is a scintillator having a garnet structure. The scintillator having a garnet structure has advantages that the scintillator is chemically stable, is neither cleavable nor deliquescent, and has excellent processability. For example, the scintillator, which is disclosed in Patent Document 1, having a garnet structure that uses luminescence from a 4f5d level of Pr3+ has a short fluorescence life time that is not longer than 40 ns.